The Houston and Dallas-Fort Worth metropolitan areas are growing toward one another as new homes and jobs spread to suburbs along Interstate 45.

On Thursday, mayors of the three major cities in the two metro areas banded together in support of a private project that's poised to connect them even more closely.

"We believe that high-speed rail connection is important to our metro areas," Houston Mayor Annise Parker said during a news conference in Houston, joined by Mayors Mike Rawlings of Dallas and Betsy Price of Fort Worth.

The three announced their support for Texas Central Railway's plan to build a bullet train system between Houston and Dallas. The project would be funded entirely by private investment, said Robert Eckels, the company's president and a former Harris County judge.

Don't expect to hop aboard for the 90-minute ride anytime soon, however. Under the current timetable, trains would start operating in 2021. Years of environmental reviews await, though the process is beginning to take shape.

Eckels said officials plan to file paperwork with the Federal Railroad Administration in the next 30 to 60 days, starting the process for a formal decision on the project.

New rail lines require federal approval even if they are privately funded.

Depending on the route and design, Eckels and others have said the 240-mile rail line would cost tens of billions of dollars. Potential investors include Japan Railway Central, whose design is being imported to the U.S.

To jump-start what has been a quiet process until the past four months, the mayors gathered Thursday to show their support. City support will be crucial as the company lays out its plans, which Eckels said include providing train service "downtown-to-downtown" between Houston and Dallas.

"We've got to get downtown," Rawlings said. "That's where the action is."

Demand for the service - to help travelers avoid time-consuming drives along crowded freeways or waits at local airports - will grow as the regions expand, officials said. High-speed rail gives travelers a choice, Rawlings said.

"There are a lot of times you want to get on a train and get there very quickly," he said.

Nearly 50,000 people travel once a week or more often between Houston and Dallas by car, according to a 2012 New York University report.

Connecting the trains to local transit service and roads remains their responsibility, the mayors agreed. Dallas has the country's largest light rail system based on miles of track, and Houston's system will grow to around 23 miles later this year.

The state is studying bullet trains between Dallas and Fort Worth. Thursday in Austin, state transportation officials appointed six members to the newly formed Commission for High-Speed Rail.

The board, created by the Texas Transportation Commission, is studying options for rail in the Dallas-Fort Worth area, but officials have stressed a private company would likely operate the service.